Hepatitis C virus (HCV) infects an estimated 400 million persons worldwide, and results in significant morbidity and mortality due to the development of chronic hepatitis in an estimated 50-80% of cases. Attempts to control infection with antiviral agents have met with limited success, and the need for an effective vaccine as well as new therapeutic strategies remains paramount. Central to the development of these approaches is better understanding of the relative contribution of the immune response in HCV infection to viral containment versus disease induction. During the funding period provided by this grant, we have performed a detailed characterization of the intrahepatic CTL response to HCV, and have now developed new and highly sensitive techniques for the quantitative assessment of immune responses in peripheral blood. Using these techniques, we have initiated a comprehensive analysis of the CTL and T helper cell response to HCV in persons with acute infection. Our results indicate that resolving acute hepatitis is associated with a vigorous and persistent CTL and T helper cell response to the virus. These results provide optimism that the immune response may be able to contain HCV infection under certain conditions, and provide the rationale for new experiments now being proposed. We hypothesize that a vigorous and broadly directed CTL response is critical to immune containment of HCV infection, and that chronic infection is the result of a persistent but inadequate immune response. We will use novel and highly sensitive immunologic assays, including elispot and flow based tetramer and intracellular cytokine assays, to perform a detailed characterization of the immune response to HCV in acute and chronic infection. By focusing our efforts on persons who control and do not control viremia, we will determine the correlates of immune protection in this infection. Specifically we propose to: a) Determine the breadth, magnitude and specificity of the CTL response in resolving and non- resolving HCV infection in humans; b) Determine the breadth, magnitude and specificity of the virus-specific T helper cell response in resolving and non-resolving HCV infection in humans; c) Evaluate the diversity and fate of clonal immune responses in acute and chronic HCV infection; and d) Determine the effects of viral sequence variation in immune escape from CTL and T helper cell responses.